A technique for welding a welded subject by irradiating the welded subject with a laser beam is known in the art. For example, a battery may include a metal cover and a metal case that are welded together through such laser beam welding. The metal cover and the metal case are welded with high accuracy to keep the battery hermetically sealed so that the electrolyte solution or the like does not leak out of the case. To increase the welding accuracy, techniques for monitoring the welding state during welding have been proposed. Japanese Laid-Open Patent Publication No. 2009-125790 describes an example of such a technique in which the welding state is monitored during arc welding.
Japanese Laid-Open Patent Publication No. 2009-125790 describes a welding monitor device that monitors arc welding. The welding monitor device includes two image capturing units that capture images around an arc-welded area, a first band-pass filter that passes wavelengths in the infrared wavelength band and is arranged in one of the image capturing units, and a second band-pass filter that passes wavelengths in the ultraviolet wavelength band and is arranged in the other image capturing unit. The welding monitor device also includes an illumination unit that has a light emission spectrum in the band passing through the second band-pass filter and illuminates the arc-welded area, an image combining unit that combines images captured by the two image capturing units, and an image display unit that displays the image combined by the image combining unit.